Method of manufacturing luminescent screens



Jan. 5, 1954 DE GIER 2,665,220

METHOD OF MANUFACTURING LUMINESCENT SCREENS Filed Feb. 26. 1949 HEATDECOMPOSABLE METAL OXIDE LAYER RTEFLECTING LAYER BEAM PRODUCING ELEMENTSINVENTOR JOHANNES DE GIER AGENT Patented Jan. 5, 1954 METHOD OFMANUFACTURING LUMINESCENT SCREENS Johannes de Gier, Eindhoven,Netherlands, as-

signor to Hartford National Bank and Trust Company, Hartford, Conn., astrustee Application February 26, 1949, Serial No. 78,507

Claims priority, application Netherlands March 5, 1948 2 Claims.

The invention relates to a method of manufacturing a luminescent screenfor a cathode-ray tube, which screen, on the side facing the electronsource, is coated with a light-reflecting metal layer. The inventionfurthermore relates to a luminescent screen manufactured by this methodand to a cathode-ray tube comprising such a screen.

It is known to coat the luminescent screen of a cathode-ray tube on theside facing the electron source with a light-reflecting metal layer.This layer must have satisfactory mirror properties on the side remotefrom the electron source and to attain this object, a variety of methodshave been suggested. According to a method which gives quitesatisfactory results, the layer of luminescent material and thereflecting metal layer are separated temporarily by an intermediatelayer which seals the granular surface of the luminescent substance. Asubstance particularly suitable for this layer being, for example,nitrocellulose. After the reflecting metal layer has been applied, thesaid intermediate layer has served its purpose and is therefore removed.This may be eiiected quite simply by heating the whole screen in anoxidizing atmosphere, for example air, the intermediate layer thusvolatilizing and being burnt. The gases thus evolved can partly escapeat the sides of the screen and Partly through the thin metal layer.

The light-reflecting metal layer must naturally be of such thinness thatit substantially does not hamper the electrons of the electron beam,since these are required to be able to strike the luminescent materialto bring it to luminescence.

It has now been found that screens manufactured in the above mannerexhibit the unpleasant property of fogging during operation of the tubein which they are housed, this being obviously due to the electronbombardment. After operation for some hours, the luminescent layerpresents a grey or brown appearance. Exhaustive experiments on whichthis invention is based showed that this fogging is related on the onehand to the method of manufacturing the screen, particularly to thetemporary provision of the intermediate layer, and on the other hand tothe light-reflecting meta1 layer. Although it is not yet quite clear towhat the fogging of the screen is due, there are indications that thefogging is produced in the following manner.

Due to the electron bombardment of the lightreflecting metal layer ametal atom of this layer may catch an electron and become a metal ion.This metal ion may travel to the luminescent layer and bring about areduction of the luminescent substance. Thus, for example, colouredmetal may be released from the luminescent sub- Stance.

An alternative explanation is that the material of the temporaryintermediate layer is not completely removed in spite of the heatingoperation and, during operation of the tube, possibly also due to theelectrons arriving at a high speed, reacts with the luminescent materialand also releases metal from the luminescent substance.

In a method according to the invention, for use in manufacturing aluminescent screen coated on the side facing the electron source with alightreiiecting metal layer, according to which a layer of luminescentmaterial applied to a support has applied to it, prior to theapplication of the lightreiiecting metal layer, a temporary intermediatelayer made of material removable by heat, the temporary intermediatelayer has applied to it a thin second intermediate layer made primarilyof one or more metal oxides. Next the temporary intermediate layer isremoved, and the lightreiiecting metal layer is then applied to thesecond intermediate layer.

The use of the method according to the invention has the advantage thatthe light-reflecting metal layer is separated by the intermediate layerof metal oxide from the luminescent layer. Metal atoms or ions of themirror layer are thus prevented from affecting the grains of theluminescent material. One of the possible above causes for fogging ofthe luminescent, screen is thus obviated. The intermediate layer made ofmetal oxides also largely obviates the second cause for fogging as setout hereinbefore, since owing to the second intermediate layer beingthin, volatilisation and oxidation of the temporary intermediate layerbecomes almost complete so that substantially no material thereof isleft. it is furthermore probable that owing to the presence of the oxidelayer any reduction of luminescent material which may be due, forexample, to materials by which the purity of the luminescent material isafiected, is prevented by the presence of the intermediate layer whichis rich in oxygen, since the reduction may be looked upon as a balancedreaction with a large quantity of a substance liable to be produced bythe reaction previously available.

The use of the temporary intermediate layer is required in order thatthe oxide layer applied may be smooth in appearance on the side facingthe electron source, a light-reflecting metal layer being thus enabledto be readily applied to the said oxide layer.

In U. S. Patent No. 2,233,786 a luminescent screen is describedcomprising a glass support to which the luminescent material is applied.On the side facing the cathode this is coated with a sealing layer ofboron-trioxide of such thickness as to be pervious to light andelectrons. The layer of boron trioxide is coated also on the side facingthe cathode with a reflecting metal layer which is pervious toelectrons. The intermediate layer of boron trioxide serves to sealsmoothly the granular luminescent layer so as to enable it to be coatedwith a satisfactory reflecting metal layer. It is not obvious how theintermediate layer of boron trioxide gets a smooth surface on the sidefacing the cathode. It is to be expected that this layer will exhibitall irregularities of the granular luminescent layer. Only if thisintermediate layer were made very thick, asubstantially smooth surfacecould probably be produced but this is inconvenient in that the output:of the screen is reduced, since an excessive quantity of electrons isretained by the intermediate layer.

Inoneembodiment of the invention the oxide layer may be applied in avery simple manner by applying a thin metal layer to the temporaryintermediate layer and by carrying out the heating operation required toremove the temporary intermediate layer in such manner that the thinmetal layer applied is substantially oxidised. Subsequent to thisheating operation the reflecting metal layer is applied to the sidefacing the electron source. Such a method can be readily carried out forexample with the metal aluminum. Heating is preferably eifected in anoxidising atmosphere, for example air.

In an alternative embodiment of the invention, the intermediate layerconstituted by oxides may be made of alumina, silica or boron trioxide.

Obviously it is not necessary for the reflecting metal layer to beconstituted by the same metal as that the oxide of which is used as theintermediate layer.

The thickness of the intermediate oxide layer is preferably chosen to beless than one quarter of that of the light reflecting metal layer. Thus,for example, if the latter layer is 0.35 micron in thickness, thethickness of the oxide layer is preferably chosen to be lower than 0.69,u..

The method according to the invention presents the following additionaladvantage.

If that part of the bulb wall which adjoins the luminescent screen mustbe coated with a metal layer, the method heretofore employed has thedisadvantage that during the heating operation for removing theintermediate layer the contact between the metal layer on the screen andthe metal layer on the bulb wall is weakened. With the method accordingto the invention, according to which the application of the reflectingmetal layer is not followed by heating the said contact is quite sureand reliable.

What I claim is:

1. A method of manufacturing a luminescent screen assembly for a cathoderay tube which comprises depositing a layer of luminescent particles ona light-transparent base, applying a temporary layer of heatvolatilizable film-forming organic coating material over saidluminescent layer to form a smooth surface thereover,

applying an electron and light-pervious layer of aluminum over saidlayer of heat-removable material, heating the aluminum layer in anoxidizing atmosphere to convert the same to a layer of an oxide of saidmetal, while removing the temporary intermediate layer, and thereafterapplying a thin light reflecting electron permeable metal layer oversaid metal oxide layer, said metal layer having a thickness which issubstantially greater than said oxide layer.

2. A method of manufacturing a luminescent screen assembly for a cathoderay tube which comprises depositing a layer of luminescent particles ona light-transparent base, applying a temporary layer of nitrocelluloseover said luminascent layer to form a smooth surface thereover, applyingan electron and light-pervious layer of aluminum over said layerofvheat-removable material, heating the aluminum layer in an oxidizingatmosphere to convert the same to a layer of an oxide of said metal,while removing the temporary intermediate layer, and thereafter applyinga thin light reflecting electron permeable metal layer over said metaloxide layer, said metal layer having a thickness which is substantiallyfour times greater than said oxide layer.

JOHANNES DE GIER.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,197,625 Teves et al Apr. 16, 1940 2,233,786 Law Mar. 4, 19412,374,310 Shaefer Apr. 24, 1945 2,374,311 Shaefer Apr. 24, 19452,378,875 Vansant June 19, 1945

1. A METHOD OF MANUFACTURING A LUMINESCENT SCREEN ASSEMBLY FOR A CATHODERAY TUBE WHICH COMPRISES DEPOSITING A LAYER OF LUMINESCENT PARTICLES ONA LIGHT-TRANSPARENT BASE, APPLYING A TEMPORARY LAYER OF HEATVOLATILIZABLE FILM-FORMING ORGANIC COATING MATERIAL OVER SAID LUMINESCENT LAYER TO FROM A SMOOTH SURFACE THEREOVER, APPLYING AN ELECTRONAND LIGHT-PERVIOUS LAYER OF ALUMINUM OVER SAID LAYER OF HEAT-REMOVABLEMATERIAL, HEATING THE ALUMINUM LAYER IN AN OXIDIZING ATMOSPHERE TOCONVERT THE SAME TO A LAYER OF AN OXIDE OF SAID METAL, WHILE REMOVINGTHE TEMPORARY INTERMEDIATE LAYER, AND THEREAFTER APPLYING A THIN LIGHTREFLECTING ELECTRON PERMEABLE METAL LAYER OVER SAID METAL OXIDE LAYER,SAID